Generally, a battery device is configured as shown in FIG. 1.
Referring to FIG. 1, the battery device includes a plurality of battery cell stacks C1˜CN, in each of which at least one battery cell is connected in series. The battery cell stacks C1˜CN are respectively connected with analog preprocessing circuits A1˜AN.
The plurality of analog preprocessing circuits A1˜AN play a role of detecting, controlling and regulating voltage and current, and a battery cell connected to one of the analog preprocessing circuits A1˜AN may be changed depending on the kind of application.
The analog preprocessing circuits A1˜AN are connected with controllers M1˜MN that control the battery cell stacks C1˜CN, respectively.
Each of the analog preprocessing circuit A1˜AN detects voltage and current of each of the battery cell stacks C1˜CN and provides it to the corresponding controller M1˜MN. Each of the controllers M1˜MN recognizes a status of each battery cell such as overvoltage, overcurrent and overdischarge according to the detection result and then turns on/off switching elements CFET, DFET so as to protect the battery device.
In the above battery device, one battery cell stack, one analog preprocessing circuit connected to the battery cell stack, and one controller connected to the analog preprocessing circuit are called ‘one unit’.
Assuming that a circuit current flowing along one of a plurality of units provided in the battery device is Icc, a circuit current of a Nth unit based on the grounding GND may be called Iccn.
The current flowing on an external load of the battery device passes in the route of CFET→DFET→Nth cell stack CN→ . . . →first cell stack→current detection resistance Rshunt, and its direction may be reversed according to charging or discharging.
In the above battery device, circuit currents Icc1, Icc2, . . . , Iccn of the first to Nth units are not always kept constantly due to unevenness of a battery cell voltage, unevenness of current in voltage and current detection circuits, and unevenness of circuit currents of the voltage and current detection circuits depending on temperature.
It may be expressed in an equation, as in the following equation 1.Icc1≠Icc2≠Iccn  Equation 1
As mentioned above, circuit currents of the first to Nth units are not identical, and such unevenness of circuit currents gives an influence on a discharging current of a cell stack and thus causes unevenness of discharging currents of the units. It is because the circuit current is added to the discharging current of the battery cell stack.
If this status is kept a long time, residual capacities of the units also become uneven as time goes.
For example, it is assumed that the first battery cell stack C1 has a residual capacity of 50%, the second battery cell stack C2 has a residual capacity of 30%, and the Nth battery cell stack CN has a residual capacity of 40%.
If a battery pack is charged with the unevenness of residual capacities of the battery cell stacks, when the first battery cell stack C1 having the greatest residual capacity is fully charged, it is determined that the entire battery device is completely charged. Thus, the unevenness of residual capacities at the beginning of charging is kept as it was.
If the charging is completed in this status, the second battery cell stack C2 has a residual capacity of 80% and the Nth battery cell stack CN has a residual capacity of 90%, so they cannot be fully charged.
In addition, the unevenness of circuit currents of the units, namely the unevenness of discharging currents by circuits, also causes unevenness of deterioration of battery cells since all units do not enter an overdischarge area at once for a long-term preservation, but some of them enter the overdischarge area fast but some of them enter the overdischarge area late.
Thus, there has been an urgent need for a technique that allows balancing circuit currents of units in a battery device.